The present invention relates to an engine idle speed control apparatus for controlling an engine idle speed by an idle control valve bypassing a throttle valve mounted on a vehicle engine, and more particularly to a control system for controlling an opening degree of the idle control valve in dependency on temperature of lubricating oil.
A conventional engine idle speed control apparatus controls the idle speed at a very low speed (e.g., 800 rpm) so that an influence of oil friction by viscosity is substantially large. It is therefore necessary to control the opening degree of an idle control valve due to the influence of the oil friction.
In view of this, there has been proposed a method of correcting the idle speed under a cool state as disclosed, e.g., in Japanese Patent Laid-open Publication No. 55-5441. According to this prior art, the opening degree of an idle control valve and the bypassing air amount are corrected in accordance with the engine coolant temperature, thereby to maintain the idle speed to constant.
In this method, the opening degree of the idle control valve is directly controlled by the coolant temperature so that the influence of oil friction cannot be eliminated reliably. Namely, even if the coolant temperature enters a warmed-up state, the viscosity of lubricating oil is still high because the oil temperature rises slower than the coolant temperature. Accordingly, the opening degree of the idle control valve which is controlled by the coolant temperature, is unnecessarily reduced resulting in lowering the engine speed or in an engine stop. Further, in the case where a learning of the opening degree of the idle control valve is performed, a learnt opening degree is largely varied according to the variation of the oil temperature so that controllability of the learning is lowered.
It is therefore necessary to consider further the oil temperature and viscosity for the control of the opening degree of the idle control valve. It can be thought of using an oil temperature sensor. However, this leads to a rise in cost and a correct temperature is hard to be detected. In consideration of the above, the oil temperature may be estimated from the coolant temperature which directly influences the former. In this case, it is preferable to use a relation between the oil temperature and the cumulative period of time while the coolant temperature is equal to or higher than a preset coolant temperature.